The transporter here envisaged comprises an endless belt which is entrained by a rotating sheave partly enveloped thereby and also passes about several deflecting rollers some of which define a stretch of that belt adjoining the path of the workpiece to be advanced by the belt in a predetemined transport direction. Such a transporter is also known, for example, from German patent No. 24 26 538.
If a crease in the workpiece or a thickening thereof, a transverse seam or some other obstacle impedes the belt motion, the driving sheave will tend to slip. Such a slippage is undesirable in any sewing machine but is particularly objectionable in the stitching of two stacked layers since, with the transporter for the other layer continuing at its normal speed, the relative position of the two layers may be unacceptably changed. It has therefore already been proposed to replace the frictionally entrained transport belts by perforated bands coacting with associated sprocket wheels; this, however, increases the space required for the driving mechanism and may even interfere with a viewing of the stitching operation since the sprockets necessarily have a larger diameter than a plain sheave. Another possible solution resides in making the belt very tight in order to intensify its contact with the sheave, yet this results in an accelerated wear and requires the use of heavier belts.